The present invention relates to coplanar transmission structures and more particularly to an improved coplanar transmission structure for suppressing spurious electromagnetic modes propagating through the substrate of the structure.
Coplanar transmission structures, such as coplanar waveguides, coplanar striplines, slotlines, and the like, are used in a wide variety of electronic applications. For example, coplanar waveguides are used in microwave wafer probes, such as described in U.S. Pat. No. 4,697,143 to Cascade Microtech, Inc. The microwave probe has an approximately triangular shaped alumina (Al.sub.2 O.sub.3) substrate on which is formed a coplanar waveguide that tapers toward the point of the triangle. Bulk microwave absorbing material containing iron or ferrite and having a high magnetic loss coefficient is secured on both surfaces of the substrate to absorb unwanted modes that propagate up the probe substrate and reflect off of the probe mounting block and back down the substrate producing resonance in the probe.
Microwave probes allow accurate on-wafer measurements of very small planar devices, such as transistors, inductors, capacitors, resistors, IC's Saw filters and the like at frequencies from DC to 100 GHz. Chip measurements of substantial accuracy can be made using microwave probes by connecting a network analyzer to a microwave probe and then calibrating the system using a calibration substrate The calibration substrate has various types of planar calibration elements formed on it, such as Line-Reflect-Line (LRL) calibration elements, Line-Reflect-Match (LRM) calibration elements, Open-Short-Load-Thru (OSL-T) calibration elements, and the like. Deviations from the ideal response of the probe/calibration substrate combination are stored in the network analyzer and software algorithms are used to compensate for these detected deviations as well as the non-ideal response of the network analyzer and the interface to the probe.
The calibration substrate is positioned on a metal chuck and is held in position by a vacuum. The chuck acts as a ground plane for the undesired microstrip modes when a signal is applied through the microwave probe. Besides the microstrip modes, surface wave modes propagate through the substrate. Recently, quartz spacers have been placed under the calibration substrate to reduce the parasitic modes generated in the calibration substrate. However, the parasitic modes still produce resonances in the incident to reflected signal ratio as measured by the network analyzer.
Attempts have been made to reduce the surface wave modes on the calibration substrate by applying a lossy material, such as nichrome, along the edges of the calibration elements. However, the dimension of the nichrome material is much shorter than the wavelength of the signal being coupled into the calibration element. Therefore, it has little effect on surface wave modes which propagate along the bottom surface of the substrate. Additionally, it has no effect on the microstrip modes generated by the metal chuck acting as a ground plane for the calibration elements.
In certain specific applications, such as yttrium-iron-garnet (YIG) oscillators, cast carbon based bulk absorbing materials is used. The carbon based bulk material has a RTV or epoxy binder, which allows it to be cast into shapes that fit into the YIG cavity. The frequency output of a YIG oscillator is dependent on the magnitude of a high magnetic field acting on YIG spheres in the oscillator cavity. The carbon based bulk material is transparent to the magnetic fields and has no affect on the operation of the YIG. Iron or ferrite magnetically loaded material, however, would disrupt the magnetic fields in the YIG. The carbon based bulk material has also been painted on lid covers of housing containing microwave circuits to suppress modes in the cavity.
Attempting to use the magnetically permeable or cast carbon based bulk microwave absorbing material, as taught in the '143 patent, under the calibration substrate would cause planarity problems between the probe and the calibration substrate. The signal and ground tips of the microwave probe must simultaneously contact the calibration substrate. The bulk absorbing material does not provide a flat uniform surface required for such precise alignment.
What is needed is a coplanar transmission structure that propagates only the mode defined by the transmission structure on a substrate and suppresses all the unwanted spurious modes that may be present in the substrate.